Apparatus for prewetting photoelectrostatic offset masters

ABSTRACT

Apparatus for applying a prewetting solution to an image-bearing surface of a photoelectrostatic master sheet, which apparatus includes a scrubber roll rotatably disposed in pressure contact with the bearing surface of a guide plate which is submersed in a prewetting solution contained in a reservoir, and two spaced sets of rotatably mounted rolls arranged to advance a photoelectrostatic master sheet from an input position where the sheet is submersed, through the nip formed by the scrubber roll against the bearing surface where the image-bearing surface of the master sheet is lightly scrubbed to remove loose toner particles and to prepare it for reproduction, and to an exit position where excess prewetting solution is removed from the surface of the master sheet.

United States Patent Touchette 1 APPARATUS FOR PREWETTING PHOTOELECTROSTATIC OFFSET MASTERS [72] Inventor: Albert Francis Touchette, Hadley,

Mass.

[73] Assignee: The Plastic Coating Corporation,

Hampshire Co., Mass.

[22] Filed: Nov. 12, 1969 [21] Appl. No.: 875,835

[52] U.S. Cl. ..355/l5, 15/77, 15/100,

95/89 R, 95/94, 118/109 [51] Int. Cl. ..G03g 15/00 [58] Field of Search ..355/l0, 15; 95/89 L, 89 A, 95/89, 94; 118/109; 15/100, 77

' FORElGN PATENTS OR APPLICATIONS Denmark ..95/89 L 1 Sept. 26, 1972 Germany ..95/89 L Great Britain ..35S/ 10 5 7 ABSTRACT Apparatus for applying a prewetting solution to an image-bearing surface of a photoelectrostatic master sheet, which apparatus includes a scrubber roll rotatably disposed in pressure contact with the bearing surface of a guide plate which is submersed in a prewetting solution contained in a reservoir, and two spaced sets of rotatably mounted rolls arranged to advance a photoelectrostatic master sheet from an input position where the sheet is submersed, through the nip formed by the scrubber roll against the bearing surface where the image-bearing surface of the master sheet is lightly scrubbed to remove loose toner particles and to prepare it for reproduction, and to an exit position where excess prewetting solution is removed from the surface of the master sheet.

17 Claims, 4 Drawing Figures PHENTEDfizs an SHEET 1 OF 2 INVENTOR. ALBERT F. TOUCH ETTE ATTORNEY.

APPARATUS FOR PREWETTING IHOTOELECTROSTATIC OFFSET MASTERS BACKGROUND OF THE INVENTION 1 Field of the Invention This invention relates to apparatus for prewetting a photoelectrostatic master carrying an image on its surface which has been formed by direct photoelectrostatic reproduction to render its normally hydrophobic background areas hydrophilic in'nature without altering the hydrophobic character of the image areas, thereby adapting the master for use as a plate for offset printing.

2. Description of the Prior Art Photoelectrostatic masters comprise a flexible base sheet which is adapted by its form and dimensions for attachment to the plate cylinder of an offset printing press. This base sheet carries on its surface a photoelectrostatic coating comprising a photo-conductive grade of zinc oxide dispersed in a matrix of a synthetic resin.

The surface of this photoelectrostatic coating is in-.

herently, hydrophobic in nature. The image which is formed on the surface of this photoelectrostatic coating is formed by a selectively deposited resinous material which is fused on the surface or which is deposited thereon-from a liquid toner, and like the photoelectrostatic coating, is inherently hydrophobic in nature. Exposed areas of the surface of the photoelectrostatic coating form the background of the image.

Various aqueous prewetting compositions have heretofore been developed for application to the surface of a photoelectrostatic master to render it suitable for use as an offset printing plate by changing thehydrophobic character of the background areas to a hydrophilic one causing them to be wettable by water, while leaving the image areas still hydrophobic and wettable with an oleaginous printing ink.

Thus the steps of producing multiple offset printed copies by the use of a photoelectrostatic master are l) reproducing an image on its surface; (2) treating the image-bearing surface with an aqueous prewetting composition; and (3) printing copies by the use of the resulting offset printing plate. The first and third of these steps are new highly developed, requiring only minutes in the production of hundreds of copies. However, the second step is relatively time-consuming and forms a bottleneck in this method of producing multiple copies.

Until recently, this second stage was carried out by manually swabbing the image-bearing surface of the photoelectrostatic master with a fibrous pad of, for example, cotton, which is moistened with an aqueous solution of a prewetting agent. This manual swabbing was time-consuming since it must be carried out with extreme care to uniformly coat the image-bearing sur-.

face with a thin film of the aqueous solution, while avoiding leaving untreated streaks on the surface, the deposition of puddles of excess solution on the surface or the marring of the image by abrasion.

An apparatus for automatically treating the imagebearing surface of a photoelectrostatic master with a prewetting solution is disclosed in US. Pat. No. 3,358,642. However this apparatus requires the use of pumps and nozzles to feed prewetting solution to the scrubbing roll. This introduced a complexity in the apparatus which is not desirable. It also presented a risk that portions of the scrubber roll might become dry and cause scratching of the imaged surface if the system failed or was out of adjustment.

It is the principal object and advantage of this invention to provide a new and improved prewetting apparatus for treatment of the image-bearing surface of a photoelectrostatic master with a prewetting solution.

It is an additional object and advantage of this invention to provide apparatus for the treatment of the image-bearing surface of a photoelectrostatic master with a prewetting solution, which apparatus is convenient to use, is rapid in operation and efficient in the application of a uniform thin film of the solution on the surface, without streaking, leaving puddles of the solution and without marring the image on the surface.

It is an additional object and advantage of this invention to provide apparatus for the treatment of the image-bearing surface of a photoelectrostatic master with a prewetting solution, which apparatus performs a scrubbing function which serves to substantially reduce or eliminate residual fused toner resin in areas where the sheet is non-imaged, thereby upgrading the quality of the resulting reproduced prints by reducing background haze.

SUMMARY OF THE INVENTION The invention constitutes an improvement over the apparatus described in US. Pat. No. 3,358,642, issued Dec. 19, 1967 to William R. Beck, and entitled Apparatus for Prewetting Photoelectrostatic Offset Masters. The apparatus of the present invention has a reservoir for containing a prewetting solution. First and second sheet transfer means are spaced apart from each other and are adapted to advance a photoelectrostatic master sheet from an input position through the prewetting solution in the reservoir and to an exit position. Guide means are disposed between the first and second sheet transfer means and provide a bearing surface. In addition, the guide means are adapted to guide the sheet from the first transfer means to the second transfer means. The bearing surface of the guide means is submersed in the prewetting solution in the reservoir. Scrubber means are disposed adjacent to the guide means and are adapted to contact the surface of a photoelectrostatic master sheet passing over the bearing surface with relative motion between the surface of the photoelectrostatic master sheet and the scrubber means.

An advantageous feature of the apparatus of the present invention is that the scrubber means and the bearing surface engage a portion of the master sheet which is submersed in the prewetting solution. This advantageous arrangement eliminates the need for any pump or associated liquid distribution assembly such as are required by the apparatus described and claimed in US. Pat. No. 3,358,642. An additional advantageous feature of the invention resulting from this arrangement is the lack of formation of streaks on the imaged surface of the master sheet due to insufficient prewetting solution since the lower portion of the scrubber means which contacts the image-bearing surface of the master sheet is submersed in the prewetting solution and is thereby always kept moist. Still further, the first sheet transfer means are also preferably submersed in the prewetting solution and allow loose particles of BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top view of a preferred form of apparatus of the invention;

FIG. 2 is a front elevation view of the apparatus shown in FIG. 1;

FIG. 3 is a sectional side elevation view taken along line 3-3 of FIG. 1; and

FIG. 4 is a sectional side elevation view taken along line 4-4 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, a reservoir is shown which is in the form of a pan or tray 10. The tray has its greater dimension extending perpendicular to the path of a photoelectrostatic master sheet through the apparatus, which path is generally designated by the dotted line labeled 11. The tray 10 has an inclining front wall 12 extending upwardly counter to the direction of the photoelectrostatic master sheet along the path 11. The wall 12 connects with the vertical front wall 13 of the tray 10 and terminates at a juncture with the horizontal bottom wall 14 of the tray 10. The opposite end of the bottom wall 14 is connected to an upwardly depending wall 15 which inclines upwardly at an angle about 45 in the direction of the path 11 of a photoelectrostatic master sheet.

The tray 10 is partially filled with a prewetting solution 16 which is preferably maintained at a substantially constant level. The preferred level of the prewetting solution 16 is such that a portion of the upper surface of the inclining front wall 12 at the forward end of the tray 10 is covered with the prewetting solution 16. This insures that a photoelectrostatic master sheet will have both of its surfaces contacted by the prewetting solution 16 when it is moved along the path 1 l and placed in engagement with the upper surface of the inclining front wall 12 which guides it into the prewetting solution 16. The upper surface of the inclining front wall 12 preferably has a plurality of raised ridges 17 aligned in the direction of the path 11 of a master sheet and spaced apart from one another so as to leave grooves 18 between them. This allows a copy sheet or master to be supported on the ridges 17 in the upper surface of the inclining front wall 12 while the lower surface of the master is contacted by the prewetting solution within the grooves 18.

First transfer means 20 are provided in the form of a pair of parallel and adjacent rolls 21 and 2 2 which contact one another to form a nip. The rolls 21 and 22 are disposed near the intersection of the vertical front wall 13 of the tray 10 and the inclining front wall 12 and slightly spaced therefrom along the path 11 of a master sheet. The rolls 21 and 22 are arranged to receive a photoelectrostatic master sheet fed along the path 11 down the inclining front wall 12 into the prewetting solution 16 and to pass the master sheet through their nip so as to advance it through the apparatus. Second transfer means are provided in the form of another pair of rolls 24 and 25 which are disposed adjacent the upper end of the upwardly depending wall 15 and along the path 11 of a master sheet. These rolls 24 and 25 are similarly arranged parallel and adjacent to oneanother to form a nip through which a master sheet is fed. The nip formed by rolls 24 and 25 is disposed above the surface of the prewetting solution 16 since they are also employed to squeegee excess prewetting solution from the master sheet as it passes between them.

Each of the rolls in each of the pairs of rolls has a structural metal core cylinder 26 and a resilient surface layer 27, which may be, for example, natural or synthetic rubber. At least one of the rolls of each pair is provided with a driving means, described below, which drives the rolls of each pair in the same direction and at the same peripheral speeds. Each of the rolls may be provided with a direct driving means, although it is usually necessary to provide only one roll of each pair with a direct drive, while leaving the second roll of each pair free to rotate under the frictional contact at the nip between the pairs of rolls.

The pair of rolls 21 and 22, forming the first transfer means 20, rotate in the direction which propels a master sheet in the direction of the other pair of rolls 24 and 25, forming the second transfer means 23, and are rotatably carried by spaced side plates 28 and 30 which are disposed in the tray 10. The first transfer means 20 are preferably located in a position beneath the surface of the prewetting solution 16 so as to insure that the master sheet is wet and washed by the solution before being contacted. This inhibits dirt or loose toner from being embedded in the master sheet. A guide bracket 31 having an inwardly depending portion 32 extending from the vertical front wall 13 toward the nip of the first transfer means 20 is attached to the vertical front wall 13. The inwardly depending portion 32 serves as a further guide for a master sheet to insure its insertion into the nip of the rolls 21 and 22.

The pair of rolls 24 and 25, forming the transfer means 23, are similarly rotatably carried by the side plates 28 and 30. These rolls 24 and 25 serve not only to draw a master sheet through the apparatus and to advance it along its path 11, but also serve to remove excess prewetting solution 16 from the master sheet immersing from the body of prewetting solution 16 in the reservoir or tray 10. The equal peripheral speed of these two pairs of rolls permits the passage of a sheet between them with neither tension on the sheet which could tear the sheet nor a buckling of the sheet between them, when different parts of the sheet are in their different nips.

The nips between these pairs of rolls forming the first transfer means 20 and the second transfer means 23 are located at somewhat different levels, to cause a sheet passing between them to travel along a generally slanting portion of the path 1 1 which may be from about 10 to about 45 and preferably from about 15 to about 21 from the horizontal plane. The purpose of this slanting portion of the path 11 is to cause the aqueous treating solution to drain from the upper surface of a sheet after it emerges from the prewetting solution 16. In the specific embodiments of the apparatus described herein, the nip of the pair of rolls 21 and 22 is at a level lower than that of the nip of the pair of rolls 24 and 25. This arrangement is preferred since it tends to cause the leading edge of a sheet passing in a somewhat upwardly direction over the guide and bearing plate to stay in contact with that plate.

The spacing between the nips of the first transfer rolls and the second transfer rolls 24 and 25, that is, the length of the path 11 between the first transfer means and the second transfer means 23, may be merely long enough to allow for the convenient location of a scrubber roll 33 between the two pairs of rolls. in any event, it should be shorter than the length of the shortest photoelectrostatic master to be treated by the use of the apparatus. In general, it is desirable to make this spacing as short as will permit convenient access to the scrubber roll 33 to keep the apparatus as compact as reasonably possible.

A first guide plate 34 is located between these pairs of rolls and has an upper bearing surface located in a position between the nips of the pairs of rolls, such that .the leading edge of a sheet leaving the nip between the entrance pair of rolls 21 and 22 will pass over the surface and be guided into the nip between the exit pair of rolls 24 and 25. The upper surface of this first guide plate 34 is desirably coated with a friction-reducing coating. The tetrafluoroethylene resin sold under the trade mark of Teflon by E. l. DuPont de Nemours & Company, Inc. and the product sold under the trade mark Ore-A-Tape by the Ore-Lube Corporation, believed to be a dispersion of molybdenum or titanium disulfide in tetrafluoroethylene resin, have been found suitable for use as antifriction layers on the upper bearing surface of this guide plate 34. The upper bearing surface of the guide plate 34 may also be covered with a sheet of a friction-reducing material such as a plastic sheet coated with one of the above-mentioned materials. Such a sheet is preferably removably attached to the plate 34 so that it may be conveniently replaced when the coating has become worn or dirty.

The scrubber roll 33 has a resilient surface and is located in a position above the guide plate 34 so that a lower segment of its surface contacts an image-bearing photoelectrostatic master passing between the entrance rolls 21 and 22 and the exit rolls 24 and 25 and passing over the guide plate 34. Scrubber roll 33 is rotatably mounted in a similar manner on journals carried by side plates 28 and 30 and positioned between entrance rolls 21 and 22 and exit rolls 24 and 25. The axis of rotation of this scrubber roll 33 is parallel to those of the entrance and exit rolls and it is provided with a positive driving means to be subsequently described which rotates it at a peripheral surface speed in excess of the peripheral surface speed at which the entrance and exit rolls are driven. The higher peripheral surface speed of rotation is conveniently achieved by making this roll 33 larger in diameter than the diameters of the entrance and exit rolls and driving it at the same axial rotational speed as the entrance and exit rolls are driven.

The direction of rotation of this scrubber roll 33 along its lower segment, which contacts a sheet passing under it and in contact with that lower segment of its surface, is the same as the direction of travel of the sheet, but the speed with which the roll moves is higher than the speed with which the sheet moves, with the result that there is a relative movement between the contacting segment of the scrubbing roll 33 and the sheet passing in contact therewith which is in the direction of the forward travel of the sheet.

The scrubber roll 33 comprises a hollow or solid cylinder 35 which carries on its cylindrical surface a layer of a synthetic sponge material 36. It is desirable to use a layer of the sponge material of substantial thickness to provide adequate resiliency in its surface. A thickness of approximately one-eighth inch has been found to provide an adequately resilient surface. Any sponge material which is inert to the aqueous prewetting solution 16 is suitable for this purpose. Polyurethane foam has been found to be particularly satisfactory. Natural and synthetic rubber foams and cellulosic foams may also be used for this purpose. The hollow or solid cylinder 35 which carries the resilient surface material may be made of any waterproof or water-resistant structural material. It is desirable to construct this roll 33 at minimal cost since it is desirable to replace it at periodic intervals when its resilient surface shows wear. It has been found that an inexpensive cylinder of waterproofed paper or cardboard paper tubing is satisfactory for the purpose.

The portions of the axles of the rolls 21, 24, and 33 extending through the side plate 30 carry drive gears 37, 38 and 40 thereon, respectively, which are interconnected by the idler gears 41 and 42 so as to control the relative speed therebetween. The drive gear 38 is connected by gear 46 to a main drive gear 43 which is operably connected to a drive motor (not shown) so that upon operation of the motor, the first and second transfer means 20 and 23 and the scrubber means 33 are driven at predetermined speeds which bear a fixed relationship to one another. The side plates 28 and 30 are connected to two spaced lifting handles 44 which permit the unit to be moved and the entire side plate assembly including the first and second transfer means 20 and 23 and the scrubber means 33 to be removed from the tray 10 when certain connecting bolts are removed, so that the tray 10 may be periodically cleaned without interference with the transfer and scrubbing mechanism. A guide plate 39 may be included to deflect the master sheet upwardly along the path 1 1. To permit adjustment of the pressure between the scrubber roll 33 and the bearing surface of the guide plate 34, the guide plate 34 is pivotably mounted between the side plates 28 and 30 and set screws 45 are provided to adjust its position with reference to flanges (not shown) affixed to the side plates 28 and 30.

In operation, a photoelectrostatic master in the form of a sheet having an upwardly directed, image-bearing surface is fed into the apparatus by directing one end of the sheet downwardly into contact with the upper sur face of the inclining front wall 12 so that it will follow the path 11. The leading end of the sheet enters the prewetting solution and advances until it is deflected off of the inwardly depending portion 32 of the guide bracket 31. This causes the sheet to be fed into the nip between the rolls 21 and 22 of the first transfer means 20 which are driven. The rolls 2] and 22 grip the sheet transversely and advance it along the path 11. Between the point where the sheet enters the prewetting solution and the point where it is gripped by the first transfer means 20, the image bearing surface of the sheet is contacted by the prewetting solution. This causes unattached and loosely attached toner particles, which form either undesirable background or the image, to be dislodged from the surface of the sheet and become suspended in the prewetting solution.

The leading end of the sheet next contacts the bearing surface of the guide plate 34 which preferably has been treated to reduce its coefficient of friction. The image-bearing surface of the sheet is then contacted by the lower segment of the scrubber roll 33 which is submersed in the prewetting solution and thereby kept moist. The scrubber roll 33 presses the sheet against the guide plate 34 and lightly scrubs the image-bearing surface of the sheet to remove any loose or poorly attached toner particles from the sheet so as to render the background clean and the image sharp. The sheet then emerges from the nip between the bearing surface of the guide plate 34 and the scrubber roll 33 and, in the embodiment shown, is deflected upwardly by the surface of the guide plate 39 toward the nip formed by the rolls 24 and 25 of a second transfer means 23. The sheet emerges from the prewetting solution prior to passing through the nip formed by the rolls 24 and 25 and excess prewetting solution is allowed to drain from its surfaces. The rolls 24 and 25 serve as squeegee rolls and remove any remaining excess prewetting solution from the surfaces of the sheet.

The apparatus described has a number of advantages over apparatus of the prior art and particularly over the apparatus disclosed in U.S. Pat. No. 3,358,642. Thus, the disposition of the scrubber means 33, such that a portion of its surface which contacts the bearing surface of the guide plate 34 is submersed in the prewetting solution 16, insures that the surface of scrubber roll 33 which contacts the image-bearing surface of the master sheet will always be moist so that the image on the master sheet is not scratched. This arrangement also prevents streaking due to dryness of the imagebearing surface of the master sheet during the scrubbing operation. The arrangement of the scrubber means 33 also insures that the scrubber means is selfcleaning so that toner particles dislodged from the imaged master sheet during the scrubbing operation will be washed from the scrubbing roll 33 and from the bearing surface of the guide plate 34 in normal operation without requiring pumps, nozzles, or relating washing apparatus.

In addition, the disposition of the first transfer means 20, such that they form a pressure nip disposed beneath the surface of the prewetting solution 16, insures that the image-bearing surface of the master sheet will not be damaged by the pressure of the rolls 21 and 22 forming the first transfer means since any loose toner particles will be washed from the surface by the action of the prewetting solution prior to passage of the master sheet through the pressure nip. In particular, this washing prevents those toner particles, which form the undesired background on the image-bearing surface of the master sheet, from being embedded into the surface of the master sheet. When the master sheet is used to form reproductions, any such embedded particles would reproduce the undesired background image. However, the arrangement in the apparatus of the in vention eliminates this problem by removing the particles before they become embedded.

From the above description of the invention, it will be apparent that various modifications in the apparatus described in detail herein may be made within the scope of the invention. For example, the second guide plate 39 might be eliminated or other guide means might be employed to govern the path of the master sheet through the apparatus. Furthermore, other forms of scrubber means might be employed with equal success. Therefore, the invention is not to be limited to the specific details of the apparatus described herein except as may be required by the following claims.

What is claimed is:

1. Apparatus for simultaneously applying a we wetting solution to an image-bearing surface of a 0 photoelectrostatic master sheet and removing partially fused toner particles therefrom which comprises a reservoir containing a prewetting solution, first and second sheet transfer means spaced apart from each other and adapted to advance a photoelectrostatic master sheet from an input I position through the prewetting solution in said reservoir and to an exit position, said first sheet transfer means comprising two rotatably mounted rolls disposed substantially parallel and adjacent to one another to form a roll nip through which said sheet is passed, said nip being disposed beneath the surface of said prewetting solution in said reservoir,

guide means providing a bearing surface, said guide means being disposed between said first and second sheet transfer means and adapted to guide said sheet from said first sheet transfer means to said second sheet transfer means, said bearing surface being submersed in said prewetting solution in said reservoir, and

scrubber means disposed adjacent said guide means and adapted to contact the image-bearing surface of a photoelectrostatic master sheet passing over said bearing surface with relative motion between the surface of said photoelectrostatic master sheet and said scrubber means for removing partially fused toner particles from the surface of said photoelectrostatic master sheet.

2. Apparatus according to claim 1, including means preceding said roll nip which provide a guide surface which terminates adjacent said roll nip, at least a portion of said guide surface being submersed in said prewetting solution.

3. Apparatus according to claim 1, wherein said guide means includes a first guide plate submersed in said prewetting solution.

4. Apparatus according to claim 3, wherein said first guide plate provides said bearing surface, and inclines upwardly in the direction of movement of said sheet through said prewetting solution.

5. Apparatus according to claim 4, wherein said first guide plate is pivotably mounted for movement about a substantially horizontal axis which is perpendicular to the path of said sheet through said prewetting solution, and including means for adjusting the angle of inclination of said first guide plate.

6. Apparatus according to claim.3, wherein said first guide plate has a bearing surface which contacts said sheet, said bearing surface having a coating of a friction-reducing material.

7. Apparatus according to claim 3, wherein said first guide plate has a bearing surface, and a sheet of friction-reducing material disposed over said bearing surface and adapted to contact said master sheet.

8. Apparatus acc'ordingto claim 1, wherein said guide means includes a guide surface along the path of said sheet through said prewetting solution, said guide surface being disposed along the path of said sheet through said prewetting solution following said bearing surface and adapted to guide said sheet into said second transfer means.

9. Apparatus according to claim 8, wherein said guide surface and said bearing surface each incline upwardly in the direction of movement of said sheet through said prewetting solution, said guide surface having a greater angle of inclination than said bearing surface with respect to horizontal.

10. Apparatus according to claim 1, wherein said scrubber means comprise a rotatably mounted cylinder having a resilient surface, the axis of said cylinder being parallel to the plane of said photoelectrostatic master sheet and substantially perpendicular to the direction of travel of said sheet through said prewetting solution.

1 1. Apparatus according to claim 10, including drive means operably connected to said scrubber means and adapted to rotate saidcylinder so that its surface contacting said sheet moves in the same direction as said sheet at a peripheral surface speed greater than the speed of the surface of said sheet.

12. Apparatus according to claim 10, wherein said cylinder carries on its cylindrical surface a layer of synthetic sponge material having substantial thickness and resiliency.

13. Apparatus according to claim 1, wherein said first and second sheet transfer means each comprise two rotatably mounted rolls disposed parallel and adjacent to one another to form a roll nip through which said sheet is passed, at least one of said rolls being rotatably driven, and wherein said apparatus includes a drive means arranged to drive at least one of the rolls of said first and second sheet transfer means at substantially equal parallel surface speeds.

14. Apparatus according to claim 13, wherein the nip of each respective set of rolls is spaced from that of the other set of rolls by a distance less than the length of the shortest photoelectrostatic master sheet to be treated.

15. Apparatus according to claim 1, including a single drive means operably connected to each of said first and second sheet transfer means and said scrubber means. v

16. Apparatus according to claim 1, including means for removing excess prewetting solution from said photoelectrostatic master sheet, said means being disposed above the surface of said we wetting solution in said reservoir and at said exit position.

17. Apparatus according to claim 16, wherein said means for removing excess prewetting solution from said photoelectrostatic master sheet comprise two rolls arranged to form a squeegee nip through which said master sheet is fed. 

1. Apparatus for simultaneously applying a prewetting solution to an image-bearing surface of a photoelectrostatic master sheet and removing partially fused toner particles therefrom which comprises a reservoir containing a prewetting solution, first and second sheet transfer means spaced apart from each other and adapted to advance a photoelectrostatic master sheet from an input position through the prewetting solution in said reservoir and to an exit position, said first sheet transfer means comprising two rotatably mounted rolls disposed substantially parallel and adjacent to one another to form a roll nip through which said sheet is passed, said nip being disposed beneath the surface of said prewetting solution in said reservoir, guide means providing a bearing surface, said guide means being disposed between said first and second sheet transfer means and adapted to guide said sheet from said first sheet transfer means to said second sheet transfer means, said bearing surface being submersed in said prewetting solution in said reservoir, and scrubber means disposed adjacent said guide means and adapted to contact the image-bearing surface of a photoelectrostatic master sheet passing over said bearing surface with relative motion between the surface of said photoelectrostatic master sheet and said scrubber means for removing partially fused toner particles from the surface of said photoelectrostatic master sheet.
 2. Apparatus according to claim 1, including means preceding said roll nip which provide a guide surface which terminates adjacent said roll nip, at least a portion of said guide surface being submersed in said prewetting solution.
 3. Apparatus according to claim 1, wherein said guide means includes a first guide plate submersed in said prewetting solution.
 4. Apparatus according to claim 3, wherein said first guide plate provides said bearing surface, and inclines upwardly in the direction of movement of said sheet through said prewetting solution.
 5. Apparatus according to claim 4, wherein said first guide plate is pivotably mounted for movement about a substantially horizontal axis which is perpendicular to the path of said sheet through said prewetting solution, and including means for adjusting the angle of inclination of said first guide plate.
 6. Apparatus according tO claim 3, wherein said first guide plate has a bearing surface which contacts said sheet, said bearing surface having a coating of a friction-reducing material.
 7. Apparatus according to claim 3, wherein said first guide plate has a bearing surface, and a sheet of friction-reducing material disposed over said bearing surface and adapted to contact said master sheet.
 8. Apparatus according to claim 1, wherein said guide means includes a guide surface along the path of said sheet through said prewetting solution, said guide surface being disposed along the path of said sheet through said prewetting solution following said bearing surface and adapted to guide said sheet into said second transfer means.
 9. Apparatus according to claim 8, wherein said guide surface and said bearing surface each incline upwardly in the direction of movement of said sheet through said prewetting solution, said guide surface having a greater angle of inclination than said bearing surface with respect to horizontal.
 10. Apparatus according to claim 1, wherein said scrubber means comprise a rotatably mounted cylinder having a resilient surface, the axis of said cylinder being parallel to the plane of said photoelectrostatic master sheet and substantially perpendicular to the direction of travel of said sheet through said prewetting solution.
 11. Apparatus according to claim 10, including drive means operably connected to said scrubber means and adapted to rotate said cylinder so that its surface contacting said sheet moves in the same direction as said sheet at a peripheral surface speed greater than the speed of the surface of said sheet.
 12. Apparatus according to claim 10, wherein said cylinder carries on its cylindrical surface a layer of synthetic sponge material having substantial thickness and resiliency.
 13. Apparatus according to claim 1, wherein said first and second sheet transfer means each comprise two rotatably mounted rolls disposed parallel and adjacent to one another to form a roll nip through which said sheet is passed, at least one of said rolls being rotatably driven, and wherein said apparatus includes a drive means arranged to drive at least one of the rolls of said first and second sheet transfer means at substantially equal parallel surface speeds.
 14. Apparatus according to claim 13, wherein the nip of each respective set of rolls is spaced from that of the other set of rolls by a distance less than the length of the shortest photoelectrostatic master sheet to be treated.
 15. Apparatus according to claim 1, including a single drive means operably connected to each of said first and second sheet transfer means and said scrubber means.
 16. Apparatus according to claim 1, including means for removing excess prewetting solution from said photoelectrostatic master sheet, said means being disposed above the surface of said prewetting solution in said reservoir and at said exit position.
 17. Apparatus according to claim 16, wherein said means for removing excess prewetting solution from said photoelectrostatic master sheet comprise two rolls arranged to form a squeegee nip through which said master sheet is fed. 